I have the same (works in -19, stays down in -20), but with the bnxt_en driver for an Broadcom Inc. and subsidiaries BCM57414 NetXtreme-E 10Gb/25Gb RDMA Ethernet Controller:
[ 11.380325] bnxt_en 0000:01:00.0 nic6: renamed from enp1s0f0np0
[...
Not wanting to be a "Just use XYZ" guy but maybe in case everything else fails you might try to switch to NFS (which the Synology typically offers too).
Check Mount Options: Look at /proc/mounts. If using cache=none or forcedirectio, try switching to default caching (cache=standard) to see if the issue stops.
Kernel Logs (dmesg): CIFS kernel drivers usually log page errors. Run "dmesg -T | grep...